Origin of potassic (C-type) adakite magmas: Experimental and field constraints

Abstract

The postcollisional, Mesozoic, C-type or high-K adakitic intrusions (K-adakites) of China lack either temporal or spatial association with subduction and have K 2O/Na 2O around unity. Otherwise, their geochemistry is very similar to ordinary adakites. New experimental data, geological observations and theoretical considerations lead us to suggest that these K-adakites were high- T (> 1050 and probably > 1075 °C), rather hydrous magmas (∼ 6 wt.% H 2O) produced by fluid-absent partial melting of metatonalites, meta-andesites, or possibly potassic metabasalts at pressures exceeding 2 GPa. Their peculiar chemistry is a consequence of both the elevated K 2O/Na 2O in the protolith and the very high pressure of partial melting. The most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths, with high mantle heat flow, during orogenic collapse.